Abstract

Diurnal patterns of hydraulic conductance of the leaf lamina
(
K
leaf
) were monitored in a field-grown tropical tree
species in an attempt to ascertain whether the dynamics of
stomatal conductance (
g
s
) and CO
2
uptake (
A
leaf
) were
associated with short-term changes in
K
leaf
. On days of high
evaporative demand mid-day depression of
K
leaf
to between
40 and 50% of pre-dawn values was followed by a rapid
recovery after 1500 h. Leaf water potential during the
recovery stage was less than
-
1 MPa implying a refilling
mechanism, or that loss of
K
leaf
was not linked to cavitation.
Laboratory measurement of the response of
K
leaf
to
Y
leaf
confirmed that leaves in the field were operating at water
potentials within the depressed region of the leaf ‘vulnerability
curve’. Diurnal courses of
K
leaf
and
Y
leaf
predicted
from measured transpiration, xylem water potential and
the
K
leaf
vulnerability function, yielded good agreement
with observed trends in both leaf parameters. Close correlation
between depression of
K
leaf
,
g
s
and
A
leaf
suggests that
xylem dysfunction in the leaf may lead to mid-day depression
of gas exchange in this species.